Timothy C. Hain, MD Page last modified: February 2, 2014
|Zodiac 901 tympanometer used at Chicago Dizziness and Hearing|
Tympanometry is one of several tests that audiologists call "immittance" tests. A tympanometer measures the acoustic immittance in the external auditory meatus as a function of air pressure within the external auditory meatus.
Acoustic "immittance" is a generic term referring to either impedance, or 1/impedance -- called admittance, according to the ASHA working group (2014). In other words, "immittance" is a vague term denoting either how easy or how hard it is for sound gets into the ear. To our understanding, you wouldn't necessarily say that "this graph plots immittance", but you might say that "this device measures immittance".
Admittance is the reciprocal of impedance (well that was helpful right ?). For those more used to electricity than sound, impedance is a measure of the opposition of a circuit to the flow of current, taking into account both static effects (called resistance) and dynamic effects (known as reactance). Dymanic means that the result is a function of frequency. Reactance may be from mass (inductors) or inverse-springs (capacitors). Stiffness is a property of springs. This whole terminology is hugely confusing, although it can be straightened out by taking a course in physics and differential calculus.
Impedance = resistance+reactance.
Admittance = 1/Impedance
Admittance (Y) is the sum of conductance (G) and susceptance (B). The conductance is the static or "real" part, and the susceptance the dynamic or "imaginary" part. Susceptance is the algebretic sum of mass susceptance and compliant susceptance. Thus it resembles reciprocal reactance.
Y = G+jB
The units of admittance are mhos (or siemens).
Although the math is all very interesting, it does not result in an especially useful understanding of what tympanometry really represents from a clinical standpoint. We think that it is most easily comprehended if one thinks of the plot as "inverse stiffness" vs. pressure). This isn't quite right, as admittance, being actually inverse impedance, is not simply 1/stiffness, but it is generally close enough.
The other "immittance" test is acoustic reflexes.Typically, both types if "immitance tests" are done using the same machine. Tympanograms can also be done using hand-held devices that will not do reflexes. These are typically used at the bedside as a quick method of determining if there is a perforation of the ear drum or a middle ear infection.
There is some overlap with AR (acoustic reflex) testing and tympanometry, and the AR technique is often used when one wants to look at pressure over time.
We like the ASHA working group paper on Tympanometry, available online. While immensely more complicated than needed for clinical work, it is very useful to establish vocabulary (AHSA, 2014).
Although there is much more to it than this, tympanometry essentially involves measuring how much sound is reflected back while pressure is swept through the ear canal. A test tone (usually 220 hz for adults and 1000 hz for children) is presented while pressure is swept through -400 to +300 dpa (deca-pascals). The amount of sound that is reflected back from the eardrum is quantified and plotted on an inverse sweep. As noted above, although not entirely accurate, we think that it is most easily comprehended if one thinks of the plot as "inverse stiffness" vs. pressure).
|A typical tympanogram plots compliance for each ear (inverse stiffness), against pressure. If the eardrum is under no positive or negative pressure, it will have it's maximum compliance at 0. On the other hand, if it is under negative or positive pressure, the peak will move to the left or right. See discussion below for more patterns.|
When the eardrum is at it's neutral position (least stiff) less sound is reflected back from the ear drum. Thus the sweep of pressure locates the ear pressure that is associated with the eardrum being neutral.
Tympanometry is mainly useful to determine if there are problems with eardrum movement. There are several patterns, which sometimes go under the name of "Liden-Jerger" (Liden, 1969; Jerger, 1970). We think that these letters are confusing as a nomenclature that incorporated the pattern (such as "flat", or displaced", etc) would seem much more useful to clinicians.
|A||Tent shaped, centered on 0||Normal|
|B||Flat, with or without high volume||Low compliance -- fluid in middle ear or perforation, or clogged tympanometer tip (with cerumen generally). The ear volume measurement can be used to figure out which one it is.|
|C||Tent-shaped, off-center.||Pressure in middle ear displaces curve to left or right. Usual cause is Eustachian tube malfunction.|
|Ac||Too high tent||Overly compliant ear drum. Possibilities include damage to the TM (most commonly), or a disarticulated ossicular chain.|
In essence one looks at the peak compliance (which should be at 0), the shape of the compliance curve (too low -- stiff, too high - -floppy), and the ear volume (too high -- perforation).
When there is a positive or negative pressure behind the ear drum, the peak of the tympanogram is displaced to one side or the other. This is nearly always due to eustachian tube malfunction.
When the eardrum is stiff due to scarring, there is less change in the trace with the pressure sweep and the tympanogram looks flat. There is also no change with pressure when there is a perforation.
|Flat tymp on the right side due to a perforation. Hypercompliant tymp on the left side due to a floppy eardrum.|
When the eardrum is floppy, perhaps due to overly exhuberant ear popping or a break in the ossicular chain, then less sounds is reflected back and the tympanogram looks "peaked" (see above).
This is all oversimplified, and there are also strange patterns that can arise due to odd physics of the ear drum that are not worth delving into.
One reason for a flat tymp is a hole in the eardrum (perforation). Flat tymps also occur in stiff ear drums. Perforations are spotted by noticing that the tymp is flat AND the ear canal volume is excessive (ECV).
|Flat tymp on the right side, in a person with perforation.|
Tympanometry also can detect abnormally stiff ear drums (often due to fluid behind them, scarring or otosclerosis), as well as abnormally floppy ear drums (called "hypercompliant"), usually due to excessive clearing of the ears.
|On the test above, the left ear is flat. This indicates that the left ear drum is abnormally stiff.|
In general, flat tymps will be due to perforation, stiffness, or a technical error (clogged tip). In perforations, the "ECV" or external canal volume number will be high. When the TM is stiff the ECV will be the same on each side (as above).
A variant of tympanometry can be used to detect eustachian tube malfunction. Someone with a working eustachian tube should be able to move their negative or postive pressure tympanometry to 0 by swallowing or "popping" their ears.
Finally, tympanometry can be used to detect abnormal contractions of the stapedius and tensor tympani muscles. This requires a machine that can run the tympanogram over 30 seconds, at a single frequency, looking for blips in the trace. Usually this is done using the acoustic reflex mode of the tympanometer.
|Rhythmic changes in impedance of the middle ear. Each bump was correlated with a high-pitched "tic" that can be heard from the outside, due to stapedius myoclonus.|
An audio recording of his myoclonus can be heard by Clicking here
The tympanogram is a very simple automated procedure that can be done by nearly anyone after about 5 minutes of training. They are insensitive to expertise - -and unlikely to be affected by technique. The person doing the tymp should be able to look into ears with an otoscope, and note whether or not there is wax or a perforation. The person doing the tymp should also realize that the machine will not work if it is clogged by wax.
Tympanograms are tests of middle ear function. They can be interpreted by persons expert in ear disease.
|© Copyright October 6, 2013 , Timothy C. Hain, M.D. All rights reserved. Last saved on October 6, 2013|